The problem of the metamorphism of rock-masses is one in which increased study has not led to unanimity of opinion. The literature of the recent Geological Congress in London suffices to remind us, how widely divergent are the conclusions to which various geologists have been led by researches in the field. Since, then, the a posteriori line gives such very different results in different hands, it may be worth while to revert for a moment to the deductive method, and try to trace the consequences, in this connexion, of admitted physical principles.